When a Spell Card is activated: Discard 1 Spell Card; negate the activation, and if you do, destroy it, and if you do that, your opponent cannot activate Spell Cards with that name for the rest of this Duel.
Bringing multiple copies of useful spell cards like Enemy Controller, or Super Rush Headlong are quite common. Cursed Seal will negate 1 of them, and make the other copies they have become a dead card.
Most people often bring multiple Enemy Controllers due to its versatile use, but they are often used to switch your monster to defense. While this can end up being bad for you if your monster's defense is low and will get destroyed easily, if you are not going to lose the fight, then it's better not to negate them. A lot of decks can have more threatening spell cards like Super Rush that destroys your monster, or ritual summoning strong ritual monsters, and whatever the new meta breaking spell cards may be, so it's better to be conservative about using Cursed Seal, even if you bring multiple of these in order to not be left with low cards in hand.
You can already make a deck around using counter traps. Monsters like Van'Dalgyon and Dark Voltanis can be special summoned when you use them, and Bountiful Artemis for more draw power. Other useful counter traps that you can bring can be Trap Jammer, or Anti-Magic Prism.
Discards for cost
Negates the activation of Spell Cards / Destroys Spell Cards
- Enemy Controller
- Machine Angel Ritual
- Super Rush Headlong
- Red Eyes Insight
- Level Up!
- Tribute to the Doom
- Cards of the Soul/Fire King Island
- Black Illusion Ritual
So its not only effective against Cyber Angels, pretty much aiming for the key Spells in most of the popular decks to cripple them. Decks relying on Fusions and Rituals face the worse fate as they probably don't have much plays after their key cards are being shut down. I don't know bout you guys but discarding 1 spell to jam these cards up sounds like a great bargain to me.